1. Technical Field
This disclosure relates to a wireless LAN apparatus supporting both DSSS and OFDM and a reception method therein.
2. Description of the Related Art
Standardization of the wireless LAN technology has progressed in IEEE 802.11 (Institute of Electrical and Electronics Engineers 802.11), and wireless LANs have been introduced to replace or be used in parallel with the conventional wired networks.
DSSS (Direct Sequence Spread Spectrum) is employed in IEEE 802.11b (hereinafter referred to as “802.11b”), and OFDM (Orthogonal Frequency Division Multiplexing) is employed in IEEE 802.11a. Further, the contents of IEEE 802.11g (hereinafter referred to as “802.11g”) are compliant with both DSSS and OFDM.
In the case of supporting both DSSS and OFDM, a DSSS part compliant with DSSS and an OFDM part compliant with OFDM are provided separately because signal processing in a baseband (BB) part to process a baseband signal differs between DSSS and OFDM although an antenna and a radio frequency (RF) part to process a radio frequency signal can be shared. Further, an automatic gain control (AGC) processing part to control the gain of the RF part, a correlation detection part to determine whether the signal is of a desired modulation method, and a demodulation part to demodulate data are provided in each of the DSSS part and the OFDM part. In general, the demodulation parts compliant with the respective methods have appropriate input gains different from each other. Therefore, AGC in the RF part should be performed independently for each of the DSSS part and the OFDM part, and cannot be performed for the DSSS part and the OFDM part at the same time.
As a conventional method in the case of supporting both DSSS and OFDM, AGC in the RF part is performed on the OFDM part side where the peak-to-average ratio of a signal waveform is high so that distortion is likely to be caused at the time of amplification, and in the DSSS part, every received signal fed thereto from the RF part is digitally amplified by a certain gain (for example, 6 dB) and input to the demodulation part.
Conventionally, as described above, AGC of the RF part is performed on the OFDM part side, while every received signal fed from the RF part to the DSSS part is digitally amplified by a certain gain in the DSSS part. However, this method has the following disadvantages.
That is, there is no problem on the OFDM part side because AGC is performed so as to control the gain to an optimal value necessary for the OFDM part. On the DSSS part side, however, the gain is not always optimal under actual conditions because the reception signal multiplied by a certain number on the OFDM side is merely fed to the DSSS part. This may result in insufficient reception performance. Further, digital amplification of the reception signal fed to the DSSS part reduces the resolution of the reception signal. This also results in insufficient reception performance.
Further, the IEEE 802.11 standards target a burst signal. A preamble signal for signal detection and synchronization is added to the head of the burst signal. The 802.11g standard supports OFDM modulation in addition to DSSS/CCK modulation employed in the 802.11b standard in order to realize the upward compatibility of the 802.11b standard and to perform high-speed communications.
In the 802.11g standard, it is specified by a protocol whether to transmit a DSSS/CCK signal or an OFDM signal in order to realize backward (downward) compatibility with the 802.11b standard. If an 802.11b terminal is in the same network, notice is given, in advance through packet transmission by a DSSS/CCK modulated signal, of time for which the network is occupied, and OFDM communications are performed within the time.
Thus, the 802.11g-compliant receiver is required to be ready to receive both a DSSS/CCK modulated signal and an OFDM modulated signal at the same time. Therefore, it is necessary to simultaneously operate both demodulators in parallel in the 802.11g-compliant receiver. It is preferable, however, that both demodulators not operate simultaneously in order to reduce power consumption as well. Therefore, it is preferable to cause only both signal detectors to operate, and after signal detection, to cause one of the demodulators selected by the detection to operate.
However, AGC control and antenna switching should also be counted in considering the operations of both signal detectors. This makes it difficult to cause all the operations of both signal detectors to be performed in parallel with each other, and RF control such as AGC should be performed sequentially.
An OFDM preamble detector can realize an initial trigger by RSSI detection when power is high. However, it is also necessary to detect a signal at high speed with a correlation detector even when the signal has low power. Therefore, in the initial signal detection, initial triggering is performed by correlation detection with the time for one symbol being 0.8 μm. Therefore, correlation processing is performed by cross-correlation. Further, the detection threshold is not too high in order to ensure generation of a trigger. That is, first of all, a trigger is generated, and then determination is made with certainty in the subsequent process.
After the initial detection, the OFDM receiver performs AGC processing at high speed using an RSSI signal value. Further, antenna switching is performed every time a burst wave is received. The AGC processing and antenna switching of OFDM should be performed in approximately 5 μs of an 8 μs period (=a short preamble period of 0.8 μs×10). When AGC is determined, detailed correlation detection, synchronization, and coarse adjustment of frequency offset are performed in the remaining approximately 3 μs.
A DSSS preamble detector for reception performs Barker code correlation processing. In general, a Barker code correlator has good noise immunity, and therefore is highly discriminative even if the input signal is an OFDM preamble signal.
The AGC processing of the DSSS part takes time because it is of a feedback type and does not use DSSS. Further, there is also the problem of frequency offset. Accordingly, the number of times of feedback should be increased if the accuracy of AGC is required.
The time for one DSSS symbol is 1 μs, and the time for one OFDM symbol is 0.8 μs. Accordingly, the OFDM detector makes determination first.
Here, it is assumed that a DSSS signal is input. The input DSSS signal has a square wave-like waveform. Since the OFDM correlator is a cross-correlator, the OFDM correlator is likely to wrongly detect a square wave-like waveform. Therefore, if wrong detection is performed on the OFDM side where the initial trigger makes determination earlier, even synchronization detection is performed after OFDM AGC/ANT processing. Thus, after detection of synchronization error, OFDM processing is switched to DSSS processing.
However, it is necessary to perform AGC processing within 20 μs from the input of the signal. Therefore, considering that the transition from OFDM processing requires 8 μsec because of synchronization error, earlier switching to DSSS processing is more desirable.